Patient Education Programme-sponsored by
Hyderabad Eye Centre
LASIK
It the most common eye operation in the world. Over a million Laser Vision Corrections of refractive errors are done in the US every year and a similar number in India.
What happens in LASIK
This is a procedure where in a cornea is made flatter to focus the external light rays on the retina. It involves making a thin cut in the cornea and applying precise laser energy to make the central cornea flatter and thereby changing the focal point of the eye. It has no effect on the lens or retina.
How does lasik Vison differ from Normal Vision
Following Lasik most patients are able to read 6/9 or better. However the quality of their vision is not the same as spectacle vision. Most patients who have refractive errors which make them spectacle dependant don’t mind that as a trade off for not needing to wear glasses to see distant objects. The main complaint that they have are
- Slight blur/ ghosting to their vision.
- Glare of head-lights at night
- Slight decreased contrast sensitivity
- Feeling of dryness in eyes
- Near vision difficulty
- Over and under correction by 0.50D
All of these improve over a period of time, some people get used to the new situation earlier than others. One must understand that these problems are due to the splitting of the cornea that has become a 2 piece tissue as against the single piece existing naturally. Over a period of time the 2 pieces blend and the brain also adapts to minimise the disability.
In some patients halos appear. This happens when the treatment zone is smaller than the pupil diameter especially in dim light. Therefore measurement of pupil diameter is always done before operating on any patient.
The above vision compromise happens in a successful lasik. Lasik is not a complication free procedure. Rare complications like infection and inflammation (DLK) can occur.
Infections are very rare and caused by rare organisms and need lengthy treatment by drops. It usually affects the eventual visual outcome by scarring the cornea.
Inflammations are usually controlled by intensive steroid drops for a period of 4- 6 weeks and do not affect the visual outcome
Flap related complications can happen in very steep or very flat corneas. These are also rare and do not affect the visual outcome of surgery.
A patient can actually help in getting a good result by focussing on the fixation light throughout the procedure. The slight over or under correction is because each eye behaves differently and each individual has different healing rate.
Wavefront Diagnostics & Custom Treatment
Summary
Wavefront-guided LASIK is a promising new technology that provides an advanced method for measuring optical distortions in the eye. Measuring and treating these distortions goes beyond nearsighted, farsighted, and astigmatism determinations that have been used for centuries. As a result, physicians can now customize the LASIK procedure according to each individual patient’s unique vision correction needs. The treatment is unique to each eye, just as a fingerprint is unique. Wavefront systems work by measuring how light is distorted as it passes into the eye and then is reflected back. This creates an optical map of the eye, highlighting individual imperfections.
Wavefront technology functions as a roadmap for LASIK surgery, providing benefits to the patient during both the evaluation and treatment process.
- During the patient evaluation process, wavefront provides the physician comprehensive individual diagnostic information, not available using earlier technologies. Thus, before surgery even begins, the surgeon is better able to determine the appropriate course of treatment.
- During treatment, wavefront allows the surgeon to tailor the laser beam settings, making the surgical procedure itself more precise. In this way, wavefront technology offers the patients sharper, crisper, better quality vision, as well as a reduction in nighttime vision difficulties, such as halos and glare.
Wavefront technology is an adjunct tool used to enhance an already safe and effective procedure. As the most common form of vision correction surgery, LASIK has already benefited millions of patients. The increased safety and the improved quality of vision benefits of customized procedures are an important technological advancement for patients and physicians alike.
Visual Errors
For purposes of this discussion, there are two categories of visual errors or “aberrations:” second-order and higher-order.
Conventional forms of optical correction have been limited to measuring the best spherical and cylindrical visual errors (second-order aberrations), which result in myopia (shortsightedness) or hyperopia (farsightedness) and regular astigmatism (blurriness), and prescribing shperocylindrical lenses in the form of spectacles, contact lenses, and conventional refractive (LASIK) surgery to correct them. Correcting second-order aberrations has the highest impact on acuity, which is the eye’s ability to distinguish object details and shape. At the same time that conventional refractive surgery corrects major, second-order spherical errors, in many cases, it also induces some degree of minor spherical aberrations.
However, about 17 percent of optical errors are higher-order aberrations. If these are minimized, image contrast and special detail are increased. Minimizing higher-order aberrations with wavefront technology by reducing the naturally occurring ones is achievable and may be particularly beneficial to individuals with unusually large amounts of higher-order aberrations.
How Wavefront Works: The wavefront aberrometer
Light can be thought of as traveling in a series of flat sheets, known as wavefronts. To clarify the confusion about light traveling as waves instead of rays, waves are just perpendicular to light rays. These light waves are wrinkled or distorted as they pass through imperfections in the eye. These errors can be displayed on a color map of the wavefront image, which is the tool that is used to diagnose, and then determine corrections, for abberrations in the eye.
There are several ways of analyzing the optical system of the eye using wavefront technology. The most common, the Hartmann-Shack wavefront sensing method, deals with light waves as they exit the eye. In this system, the surgeon or other professional shines a small, low-intensity laser into the eye, and the patient focuses on the light. As that light scatters off of the retina (the rear-most portion of the eye) it passes through the lens, the rear surface of the cornea (the clear, crystalline front part of the eye) and the front surface of the cornea. Thus, the emerging waves of light are distorted by the imperfections in the total visual system of the eye. After leaving the eye, the light passes through an array of many small lenses in the sensing device (called an aberrometer), and is focused into spots, which are recorded by a special camera. The deviation of the spots from their ideal location provides information about focusing imperfections in the visual system.
Wavefront-Guided Treatment
The goal of wavefront-guided laser treatment is to make corrections in the surface of the cornea that compensate for errors in the total visual system. Thus, the amount of wrinkle or error in the wavefront reflected from the back of the eye, as compared to the reference wavefront that was projected into it, defines the compensating optical correction. If the wavefront is retarded in relation to the reference wavefront, the laser must remove more tissue from the part of cornea related to that pattern. If the wavefront is advanced (in front of the referenced wavefront), the laser must remove less tissue. It should be noted that wavefront treatment does induce some minor second-order spherical errors, but to a significantly lesser extent than conventional refractive surgery.
In this way, a wavefront-guided treatment is customized to the characteristics of each eye and intended to minimize higher-order aberrations so that the greatest quality of vision can be achieved.
Wavefront technology is relatively new to the United States. The U.S. Food and Drug Administration (FDA) issued its first approval of a wavefront system in August 2002, and other major US laser manufacturers are expected to receive their approvals in 2003. As the FDA approves systems and they become widely available, patients will have greater access to wavefront technology and treatment.
Presurgical Considerations for Lasik
A wide range of refractive errors including low-to-high myopia, low-to-moderate hyperopia, and large amounts of astigmatism can be easily corrected by LASIK. In addition, many common systemic disorders such as diabetes are no longer a contraindication to surgery. Table 1 lists specific criteria for patient selection for the LASIK procedure.[6,7]
The FDA has approved LASIK in people 18 years and older with stable refractions. Some contraindications include pregnancy and having had eye diseases in the past such as ocular herpes, cataract, glaucoma, and keratoconus. In addition to a thorough preoperative examination and counseling of any patient interested in refractive surgery, some additional testing is warranted to help screen for possible postoperative side effects.
Pupil Examination
The pupil should be measured in both scotopic (dim) and normal (photopic) light. Measurements may be obtained with infrared pupillometers or with a simple pupil card.[8] There is a possibility that young patients with large scotopic pupils may be at higher risk for developing symptoms of glare, halos, and nighttime visual disturbances following refractive surgery. Today, with large ablation zones and the use of aberrometers for custom ablations, this is less likely.
Dry Eye Testing
Dryness after LASIK is one of the most common symptoms reported by patients.[9] It is believed to result from the cutting of the anterior flap and the loss of cornea sensation that eliminates reflex tearing. Proper preoperative testing evaluating the precorneal tear film, the use of artificial tears before surgery, and discussions about this common condition with the patient should be documented. The prescription eye drops Restasis (cyclosporine ophthalmic emulsion 0.05%) can also be used.
Topography
It is critical for all potential refractive surgery candidates to undergo preoperative cornea mapping (topography) and cornea thickness (pachymetry) measurements to exclude patients at risk who may respond poorly to the surgery.[7] Many computerized machines are available to analyze the anterior and posterior curvature of the cornea to aid in selecting and screening good candidates for surgery. These instruments map the surface of the cornea and generate data, which help eliminate patients with abnormal corneas such as seen in keratoconus (a thinning disorder of the cornea that changes it from a round shape to a cone.)
Pachymetry
Pachymetry is a measurement of the thickness of the cornea. The average cornea thickness is approximately 540 microns. Preoperative cornea thickness is an important measure used to determine the stromal thickness that will result from subtracting the flap thickness and the amount of tissue removed by the treatment.[10] It is crucial to leave behind enough corneal thickness to maintain cornea integrity so that the cornea does not become ectatic (see Cornea Ectasia andCollagen Cross-Linking). Cornea flap thickness can range from 110 to 180 microns. The minimum residual stromal thickness is recommended as no less than 250 microns.[11]
Types of Lasik Surgery
The patient is conscious during surgery but can be given mild sedation with an oral benzodiazepine such as diazepam. Anesthetic ophthalmic drops (e.g., proparacaine) are given immediately before the drapes are applied and the speculum inserted. The patient is asked to look directly at a fixation target under the laser (patient cooperation is very helpful).
LASIK involves photoablation (pulses of UV light energy) of the corneal stroma after a cornea flap has been created. The laser energy can be programmed to flatten the central cornea to correct myopic errors (nearsightedness) or steepen the peripheral cornea to correct hyperopic errors (farsightedness). Traditionally, flaps have been produced with mechanical microkeratomes, but more recently femtosecond laser technology has emerged as an alternative for flap creation.[12] This procedure uses ultrafast lasers (all-laser LASIK) that precisely photodisrupt tissue by using very short duration energy pulses. The precision of this technique has resulted in wide acceptance of the technology.
Conventional LASIK
Conventional LASIK does not use aberrometry (measurement of the imperfections in the optical system of the eye) when correcting the refractive error.[13] Treatment is based upon using the refraction from responses obtained with a phoropter and recent eyeglass prescription. This corrects only low-order aberrations known as sphere and cylinder. The data are manually entered into the laser.
Wavefront-guided LASIK
Wavefront aberrometry has redefined the diagnosis and treatment of refractive errors in the 21st century. This technology allows the capturing and measurement of the total optical system of the eye. Aberrometers using advanced polynomials are capable of measuring higher-order imperfections smaller than the wavelength of light and using this information to reduce imperfections produced by the application of the excimer laser.[14]
Studies have shown that precise wavefront-guided ablation minimizes postoperative higher-order aberration, resulting in better contrast sensitivity compared to conventional treatment.[15] Patients theoretically have better nighttime vision and less chance of needing an enhancement. This has been promoted as high-definition LASIK utilizing "optical fingerprints" made possible by wavefront aberrometry.
Mechanical Keratome
In this procedure, an automated, precision gear-driven machine with an oscillating blade is used to make a thin (110–160 micron) superior hinged flap on the anterior surface of the cornea. This is done once the eye is immobilized with a low-pressure suction ring. Complications during translation of the keratome are uncommon but can be a major concern. Incomplete flaps are easily handled but prevent the application of the laser until a later time.
Femtosecond Laser (All-laser LASIK)
More recently, femto-second laser technology (all-laser LASIK) has emerged as an alternative to mechanical flap creation.[16] This method precisely photodisrupts tissue with short-duration energy pulses that cleave cornea tissue at a predetermined depth, forming bubbles of water and carbon dioxide at a plane that allows for a smooth interface once the flap is lifted.[17] There appears to be no difference between femtosecond and microkeratome flap creation as far as visual outcomes. However, femtosecond flaps can be made thinner and their parameters more precise than conventional methods. There is also an expectation of greater safety to the patient than with mechanical devices.
Photorefractive Keratectomy (PRK)
Laser refractive surgery procedures can either be performed by intrastromal ablation (LASIK) or by surface ablation, generally known as PRK. Both use the excimer laser.
Experimental studies evaluating the excimer laser with its UV light energy began in the early 1980s. In the mid 1980s, animal and human applications performed at Louisiana State University in New Orleans demonstrated its potential for correction of refractive errors. Many FDA clinical trials have resulted in the approval of the excimer laser to reduce or eliminate a wide spectrum of refractive errors.[13]
Surface applications (PRK) have gained a small resurgence in patients with thin corneas and in poor candidates for LASIK who may be at risk for ectasia (a bulging of the cornea). Management of postoperative pain and the application of the chemotherapeutic drug mitomycin C (MMC) to prevent haze and scarring have added to PRK's safety and popularity.[18-20]
Complications
Intraoperative complications related to the use of the microkeratome occur at a rate of less than 1% in experienced hands and usually result in an incomplete or partial primary cut.[21–23] When the flap appears less than ideal, it should be replaced, and the procedure can be successfully repeated in 2 to 3 months.
Dry Eyes
There is a high incidence of the development of dry eyes after refractive surgery (up to 36.36%).[24,25] This symptom may last up to 6 months after surgery or be permanent. A proper preoperative evaluation for dry eyes is necessary to help prevent this postoperatively Pharmacologic management of dry eyes includes prescription and nonprescription medications ( Table 2 ). Another therapy is punctal occlusion, where a collagen plug is placed in the natural drain of the eye.
Cornea Ectasia and Collagen Cross-linking
Ectasia (protrusion) can follow LASIK when the cornea takes on an appearance similar to keratoconus. Visual acuity is reduced, and glasses or soft contact lenses can no longer improve vision. Ectasia is a serious complication with certain preoperative risk factors, including abnormal cornea topography and thin corneas.[26]
By combining a solution of riboflavin and UV light exposure, corneas that have suffered ectasia are being treated to strengthen their weakened status. The procedure is a strengthening of the cross-links of the collagen fibers to allow stability.[27,28] Under topical anesthesia, the central area epithelium is removed. The riboflavin is topically applied for a period of minutes followed by irradiation with intense 365-nm UVA light for approximately 30 minutes. The process has been compared to snow blindness that occurs after UV exposures.
Presently, there is a multicenter FDA clinical trial analyzing the safety and efficacy of cross-linking for postoperative ectasia.[29] Other clinical studies have shown that it may be just as effective to inject the riboflavin directly into the stroma without removing the epithelium.[30] Cornea cross-linking may be helpful in treating keratoconus and in strengthening borderline corneas in patients who are interested in refractive cornea surgery but have suspect or slightly distorted corneas.